The medial nucleus of the trapezoid body is an essential sign-inverting synapse in the binaural sound localization circuit. The major synapse in the MNTB is the calyx of Held, a. glutamatergic axosomatic synapse, with a uniquely large presynaptic terminal. This morphology allows paired pre-and postsynaptic intracellular recordings, allowing real-time measurements of presynaptic vesicle release, and postsynaptic receptor responses. The calyx shows activity-dependent functional synaptic plasticity, and also undergoes well-characterized postembryonic developmental changes. There are several modulatory transmitters and receptors present at the calyx of Held, that likely play a role in shaping the synpatic properties during development. 0f these, the metabotropic glutamate receptors, (mGluRs) are enigmatic, as they do not to date show a well-defined physiological role in either synaptic plasticity, or synaptic development. My interest lies in investigating the functional role of the group Ill mGluRs, which are located presynaptically at the calyx of Held. Using a combination of immunohistochemistry, electrophysiology, and murine genetics, I will determine the identity and role of group Ill mGluRs in synaptic plasticity and development of the calyx.